If the device identified by GRUB as the boot device contains a ZFS storage
pool, the menu.lst file is used to create the GRUB menu.

On an x86 based system with multiple ZFS BEs, you can select a BE from
the GRUB menu. If the root file system corresponding to this menu entry is
a ZFS dataset, the following option is added:

-B $ZFS-BOOTFS

Example 5–10 x86: Booting a ZFS File System

When a system boots from a ZFS file system, the root device is specified
by the boot -B $ZFS-BOOTFS parameter on either the kernel or module line in the GRUB menu entry. This
parameter value, similar to all parameters specified by the -B option,
is passed by GRUB to the kernel. For example: